1. Field of the Invention
The present invention relates to a method and an installation for rehabilitating the subgrade supporting a ballast bed for a track, wherein ballast from the ballast bed is comminuted to obtain gravel and the gravel is discharged on the exposed subgrade to produce a protective layer for the subgrade.
2. Description of the Prior Art
German utility model No G 93 06 779.8, published Aug. 26, 1993, discloses a track-bound car for processing ballast. This car is equipped with a rock breaking device for comminuting ballast and a screening device for separating the comminuted ballast into portions of coarser and finer gravel. The finer gravel portion is discharged as gravel for the subgrade and the coarser gravel portion is conveyed to an end of the car for storage in an adjoining freight car.
German patent application No. 4,301,182, published Jul. 21, 1994, describes a method of rehabilitating a ballast bed by removing ballast from the bed and at least partially replacing it with new ballast. The removed ballast is classified in grain sizes useful for a protective layer material for the subgrade and in grain sizes exceeding that range.
U.S. Pat. No. 4,479,439 discloses a method and installation for improving a track bed, wherein a protective layer of a gravel-sand mixture is laid between the subgrade and the ballast in a continuous manner. The installation comprises a vertically adjustable ballast excavating device mounted between two undercarriages and a conveyor arrangement for receiving and discharging the excavated ballast. A discharge end of a conveyor arrangement for the sand forming the protective layer is arranged immediately behind the ballast excavating device, in the operating direction. A discharge end of a further conveyor arrangement for new ballast is arranged behind the discharge end of the sand conveyor arrangement. The track is lifted off the track bed about centrally between the undercarriages, and the encrusted ballast below the lifted track is continuously removed by the excavating device. At the same time, sand is continuously discharged on the track and is laid on the exposed subgrade as it falls between the track ties. The protective layer is compacted and the new ballast is discharged on the compacted layer and planed.
Another method and installation for improving a track bed is described in U.S. Pat. No. 4,357,874. This installation comprises two independently advancing track working machines. Each machine comprises a bridge-like machine frame which, at one end, has an off-track undercarriage arranged between two on-track undercarriages. Subgrade rehabilitation is initiated by the first machine by lifting an assembled track section to expose a trackless renewal section. The second machine then removes the encrusted ballast from the trackless renewal section, and the first machine is used again to lay a protective sand layer on the subgrade and new ballast on the protective sand layer. This system is particularly useful for the rehabilitation of short track sections.
U.S. Pat. No. 5,257,580 discloses yet another arrangement and method for producing a protective layer on the subgrade of a track. This installation, too, has two independent track working machines, the first one of which is equipped with a ballast excavating device and the second one of which carries a track lifting device as well as a planing and compacting device.
Another method for rehabilitating a track bed subgrade is known from UIC-Codex 722 E of the Internationaler Eisenbahnverband (International Railroad Union), page 11, dated Jan. 1, 1990. According to this method, encrusted ballast is completely removed by a ballast cleaning machine in a first .step. The excavated ballast is partially stored on the shoulder or in suitable silo cars. In a subsequent step, the exposed subgrade is planed, a textile layer is optionally placed on the exposed subgrade, and the track is laid thereon. Subsequently, a gravel-sand mixture is discharged from the car and is leveled by a planing device. Afterwards, a track tamping, leveling and lining machine is moved into position to lift the track and to tamp the gravel-sand mixture under the ties with specially enlarged tamping picks. Thereafter, the track is raised, ballast is discharged, and the ties are tamped in conjunction with a correction of the track position.